vWF is a glycoprotein circulating in plasma as a series of multimers ranging in size from about 500 to 20,000 kD. Multimeric forms of vWF are composed of 250 kD polypeptide subunits linked together by disulfide bonds. vWF mediates the initial platelet adhesion to the subendothelium of a damaged vessel wall, though only the largest multimers appear to exhibit haemostatic activity. Such vWF multimers having large molecular masses are stored in the Weibel Palade bodies of endothelial cells, and it is believed that endothelial cells secrete these large polymeric forms of vWF. Those forms of vWF which have a low molecular weight (low molecular weight or LMW vWF) are believed to arise from proteolytic cleavage of the larger multimers.
A small portion of the vWF present in normal plasma circulates as 189, 176 and 140 kD fragments resulting from proteolytic degradation of vWF in vivo, the 140 kD fragment being derived from the N-terminal region, and the 176 kD fragment from the C-terminal region of the subunit. When LMW forms of vWF are isolated from normal human plasma and subjected to SDS-PAGE (polyacrylamide gel electrophoreses) after disulfide reduction, an unusually high portion of vWF fragments are found. This finding is compatible with the view that LMW forms of vWF have been partially or predominantly derived from large multimers by proteolytic degradation.
The proteolytic degradation of vWF is a physiological process in healthy individuals, yet in patients suffering from von Willebrand disease (vWD) type 2A it may be accelerated, and as a consequence these patients lack the vWF multimers with the largest molecular masses. A lack of large vWF multimers and an increased level of proteolytic fragments are also observed in acquired von Willebrand disease (vWD) associated with myeloproliferation syndrome, indicating increased in vivo proteolysis in this condition as well.
In patients with thrombotic thrombocytopenic purpura (TTP), on the other hand, unusually large vWF mummers are detected, and increased vWF binding to platelets has been demonstrated in these patients (Moake et al., New Engl. J. Med., 1982, 307, pp. 1432-1435). Familial TTP is associated with a severe congenital deficiency of vWF protease, while the presence of vWF-cleaving proteases inhibiting autoantibodies has been observed in patients with non-familial TTP.
The large multimers of vWF associated with TTP normally disappear after a patient is transfused with normal fresh frozen plasma. Presently, plasma exchange is the most important treatment for TTP, although significant side effects have been reported with this therapy. The existence of a severe congenital deficiency of vWF protease has been established in patients with familial TTP and the presence of a vWF-cleaving protease inhibiting autoantibodies has been observed in patients with non-familial TTP.
Several proteases have been shown to be able to cleave vWF, thereby impairing its binding affinity for platelets. However, in vitro the cleavage of vWF with these proteases in each case results in cleavage products different from the fragments derived from in vivo cleavage.
Thus, for example, while plasmin is capable of cleaving several peptide bonds in vWF, plasmin-treated vWF retains a high molecular weight core region retaining about 70% of its platelet agglutinating activity (determined as ristocetin cofactor). A 34 kD peptide is split from the N-termini of individual vWF subunits in the early stages of plasmin treatment, and epitope mapping of such plasmin-induced fragments show that these fragments originated from regions of the vWF subunit that are different from the vWF fragments present in circulating plasma.
Porcine pancreatic elastase and various serine proteases released from human leukocytes have also been shown to degrade vWF proteolytically with a resultant loss of large multimers. Epitope mapping of the degradation products again indicates that these fragments also differ from those present in normal plasma and in vWD type 2A. In addition, a calpain-like protease released from human platelets has been shown to degrade large vWF multimers and to create vWF fragments similar to those observed in vivo.